Instruments which detect thermal radiation are well known. Radiation from the human body or from other objects can easily be detected by infra-red detecting instruments.
These instruments operate in the atmospheric transparency windows of 3 to 5 micrometers and 8 to 12 micrometers. Infra-red imaging at wavelengths outside of these windows is not practical due to atmospheric absorption. In images obtained with these devices, objects with high emissivities and objects having a higher temperature relative to the background appear as bright silhouettes. This is due to the emitted power of these objects. The emitted power is described by the equation: EQU W=.epsilon..sigma.T.sup.4
where W=emitted power in BTU/hr.-ft..sup.2 .epsilon.=emissivity, .sigma.=the Stephan-Boltzman constant, and T=temperature in degrees Rankine.
From this equation it can be seen that there are two possible approaches to subdue a thermal image: use low emissivity materials on the exterior surface or reduce the exterior surface temperature. The typical approach is to use low emissivity materials on the exterior surface and then to cover the low emissivity surface with materials which are transparent in infrared (IR) wavelengths but optically opaque to provide visual camouflage. The second approach is to use thermal insulation to reduce the exterior surface temperature. Another option is a combination of these methods.
It has long been a desirable goal to develop materials that protect personnel or equipment from detection by electromagnetic, and especially infra-red, detecting equipment without detracting from the mobility of the personnel or equipment.
For example, U.S. Pat. No. 5,281,460 issued to Cox provides a pattern of strips attached to a porous nylon mesh. The strips are coated with silver, copper, or pigment.
U.S. Pat. No. 4,495,239 issued to Pusch et al. employs a base layer of textile fabric having a vapor deposited metallic reflecting layer on it, followed by a camouflage paint.
U.S. Pat. No. 4,659,602 issued to Birch employs a woven material that has a metal foil on it and a polyethylene sheet containing a conductive particulate.
In U.S. Pat. No. 4,621,012 issued to Pusch, a textile is coated with a thermoplastic that has selected dipole material in it. The material has a metallic layer to reflect infra-red.
U.S. Pat. No. 4,467,005 issued to Pusch et al. employs a support netting with a carrier web on each side having an infra-red reflecting metal coating. The material is water vapor permeable.
U.S. Pat. No. 4,533,591 issued to Sorko-Ram provides a thermoplastic resin having discrete electromagnetic particles dispersed in it.
U.S. Pat. No. 4,064,305 issued to Wallin provides a knit formed of strands of noncontinuous polymeric fibers and noncontinuous metal fibers which reflect radar waves.
U.S. Pat. No. 4,529,633 issued to Karlsson teaches an electromagnetic reflecting material made of a layer of polyethylene, a layer of a metal coating, an adhesive, and a fabric.
Because of the presence of plastic layers, the compositions of the patents do not allow water vapor to escape easily and, when worn as garments, are uncomfortable or when draped over equipment cause "sweating" of the equipment. One exception is U.S. Pat. No. 4,467,005 which claims water-vapor permeability, but not air permeability. However, to a person skilled in the art it is readily apparent that the technique described to achieve water vapor permeability and waterproofness would not result in a sufficiently high water vapor permeability to be of any practical value. Any improvements in water vapor permeability would result in a corresponding reduction in waterproofness. The materials described in the aforementioned patent provide a satisfactory surface for metallization and are acceptable for uses where a high degree of flexibility and mobility are not required, such as a covering for stationary objects, but many disadvantages surface when these materials are used to provide thermal imaging protection for an individual person. Chief among these disadvantages are the lack of drape, low moisture vapor permeability, and weight. In addition to the aforementioned disadvantages, the metallized surface is on the exterior of the laminates where it is in a position to be damaged or scraped off while moving through brush.
It is desirable from a physiological standpoint to reduce the heat stress of the person wearing infra-red camouflage garments to the largest extent possible. This can be accomplished by increasing the evaporative cooling of the body by allowing moisture vapor to easily permeate through the laminate, and by reducing weight and thickness of the total thermal camouflage package.
Another exception is disclosed in U.S. Pat. No. 4,557,957 issued to Manniso which teaches a hydrophilic metal coating on microporous expanded polytetrafluoroethylene membranes. Although laminates constructed using the coated membrane described in this patent offer some advantage in thermal and physiological performance over the other materials described above, it would not be sufficiently waterproof to be of any practical value. As a result, the metal layer as disclosed by this reference is subject to corrosion and abrasion.